It is known that aluminosilicophosphate (SAPO) molecular sieves can be used as the catalyst for converting lower carbon oxygenates such as methanol and/or dimethyl ether to lower olefins such as ethylene, propylene and butylene. Hereinto, a series of SAPO molecular sieves such as SAPO-5, SAPO-11, SAPO-17, SAPO-41, SAPO-34 and SAPO-41 have been developed as catalysts, e.g. for converting oxygenates to olefins, and it is well known that SAPO-34, when being used for producing olefins from methanol and/or dimethyl ether (MTO), has excellent catalytic properties due to small pore diameter and good hydrothermal stability.
Producing olefins from oxygenates is effected primarily by catalytic cracking, which is generally an exothermic process. Specifically, as to producing lower olefins such as ethylene, propylene and etc. from methanol and/or dimethyl ether, the targeted product is ethylene and propylene, however, during the process butylene, pentene, hexene as well as their corresponding alkanes are produced at minor amounts too, this is because during the process, in addition to that methanol and/or dimethyl ether being catalytically cracked to olefins, the produced olefins may further subject to secondary reactions such as conversions between each other, e.g. ethylene and/or propylene may further oligomerize to C4+ olefins.
Thus, in order to increase the production of ethylene and propylene, not only the overall conversion of the process has to be improved to convert the reactants as much as possible, but also the overall selectivity to ethylene and/or propylene has to be improved too. Thus, for certain SAPO catalysts, the reactants have to contact with the catalyst sufficiently to get converted as much as possible, however, the product gases have to contact with the catalyst as little as possible to avoid or minimize secondary reactions such as the oligomerizations of ethylene and/or propylene to higher olefins.
Regarding the reactions for producing olefins from oxygenates such as MTO, there are some reactors being developed in the prior art, including dense phase fluidized bed reactor and riser. For example, CN1166478A disclosed a process for producing lower olefins such as ethylene, propylene and etc. from methanol or dimethyl ether, wherein SAPO-34 molecular sieve is used as the catalyst to carry out reaction and gets regenerated continuously in a dense phase circulation fluidized bed reactor; U.S. Pat. No. 4,547,616 disclosed a continuous process for producing lower olefins from oxygenates using a turbulent fluidized bed, wherein the turbulent fluidized bed is also a dense phase fluidized bed reactor; and U.S. Pat. No. 6,023,005 disclosed a process for converting oxygenates to olefins in the presence of molecular sieves as catalyst, wherein a riser is used as the reactor.
As to the dense phase fluidized bed reactor, a heat removing means may be incorporated into the bed so that the reaction temperature can be controlled easily, however, due to a serious gas and solid backmixing in the dense phase zone, a big catalyst inventory is necessary to ensure the conversion of the feed and at the same time a bigger settling chamber is necessary to separate the catalyst from the product gases, thus, there is a great chance for the secondary reactions, which is not favorable to the overall selectivity to ethylene and propylene during the process.
As to the riser, due to the gas and solid traveling upwardly co-currently with less backmixing, the catalyst inventory may be reduced, however, it is not easy to control the reaction temperature in the riser; furthermore, due to the slower reaction velocity from oxygenates to olefins, it is hardly to convert the feed completely by riser reactor.
U.S. Pat. No. 6,166,282 disclosed a fast fluidized bed reactor for MTO process, which comprises an upper separation zone and a lower reaction zone, wherein the reaction zone comprises a dense phase zone and a transition zone above the dense phase zone, and the reactants are further converted completely after being reacted in the dense phase zone. Compared with the conventional bubbling bed, the fast fluidized bed significantly reduces the reactor size and the catalyst inventory and thus saves the cost, however, the gas and solid backmixing problem is still present and the gas entering into the settling chamber needs more time to enter into the cyclone, thus, this gas may still subject to secondary reactions, which is not favorable to the overall selectivity to ethylene and propylene.
Thus, some further improvements are still needed for the reactor and process for producing olefins from oxygenates, in order to improve the conversion of the reactants as well as the selectivity to the products.